Recently, we reported the discovery of three novel coronaviruses, bulbul coronavirus
HKU11, thrush coronavirus HKU12, and munia coronavirus HKU13, which were identified
as representatives of a novel genus, Deltacoronavirus, in the subfamily Coronavirinae.
In this territory-wide molecular epidemiology study involving 3,137 mammals and 3,298
birds, we discovered seven additional novel deltacoronaviruses in pigs and birds,
which we named porcine coronavirus HKU15, white-eye coronavirus HKU16, sparrow coronavirus
HKU17, magpie robin coronavirus HKU18, night heron coronavirus HKU19, wigeon coronavirus
HKU20, and common moorhen coronavirus HKU21. Complete genome sequencing and comparative
genome analysis showed that the avian and mammalian deltacoronaviruses have similar
genome characteristics and structures. They all have relatively small genomes (25.421
to 26.674 kb), the smallest among all coronaviruses. They all have a single papain-like
protease domain in the nsp3 gene; an accessory gene, NS6 open reading frame (ORF),
located between the M and N genes; and a variable number of accessory genes (up to
four) downstream of the N gene. Moreover, they all have the same putative transcription
regulatory sequence of ACACCA. Molecular clock analysis showed that the most recent
common ancestor of all coronaviruses was estimated at approximately 8100 BC, and those
of Alphacoronavirus, Betacoronavirus, Gammacoronavirus, and Deltacoronavirus were
at approximately 2400 BC, 3300 BC, 2800 BC, and 3000 BC, respectively. From our studies,
it appears that bats and birds, the warm blooded flying vertebrates, are ideal hosts
for the coronavirus gene source, bats for Alphacoronavirus and Betacoronavirus and
birds for Gammacoronavirus and Deltacoronavirus, to fuel coronavirus evolution and
dissemination.